its with the old sencha command 2.0.2..
i drilled down into the build routine and found that the phantom.js process crashes for no apparent reason. running the same phantomjs command from the command line standalone shows that it dumps the dependencies as expected, then crashes.
i managed to work around this problem by fixing \sdk\command\vendor\phantomjs\dependencies.js to ignore the crash, but it would still be interesting to find what is causing this..
i also tried updating the phantomjs binary with the latest version, but after that it would not crash but the phantomjs binary seems to be caught in an endless loop after the output...

since it looks like i am the only one having this problem, it does not seem worth more effort investigating.
anyway, thanks mitchell!